Plant hormones — Edexcel GCSE Biology
How auxins control phototropism and gravitropism, and the commercial uses of auxins, gibberellins and ethene.
Revision notes
2 short notes, in the order of the specification. Each one in short:
A tropism is a growth response to a stimulus, and auxins are the plant hormones involved in phototropism and gravitropism. In a shoot lit from one side, auxin moves to the shaded side and makes those cells elongate, so the shoot bends towards the light. In roots, auxin slows growth on the lower side, so roots grow downwards.
Commercial uses of plant hormones
Higher tierPlant hormones are used commercially to control how plants grow. Auxins are used as selective weedkillers, which make broad-leaved weeds grow uncontrollably, and in rooting powders for cuttings. Gibberellins end seed dormancy, make flowers and fruits develop and produce seedless fruit. Ethene is used to ripen fruit that was picked unripe for transport.
Exam questions
5 exam-style questions (11 marks), each with its mark scheme.
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8 cards: flip them, mark what you knew, and practise the rest.
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The whole of plant structures and their functions on one page, so you can see where this subtopic fits.
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Quick check questions
What is a tropism?
Show answer
A growth response of a plant to a stimulus.
Which hormone is involved in phototropism and gravitropism?
Show answer
Auxin.
Why does a shoot bend towards light?
Show answer
Auxin moves to the shaded side and makes those cells elongate more, so that side grows faster.
Are roots positively or negatively gravitropic?
Show answer
Positively gravitropic: they grow downwards, in the direction of gravity.
Frequently asked questions
How do auxins cause phototropism?
Auxin is made in the tip of the shoot. When light shines from one side, auxin moves to the shaded side, where it makes the cells elongate. The cells on the shaded side grow faster than those on the lit side, so the shoot bends towards the light. Shoots are therefore positively phototropic.
Why do roots grow downwards?
Roots grow downwards because they are positively gravitropic. In a root lying on its side, auxin collects on the lower side, where it slows the growth of the cells. The cells on the upper side grow faster, so the root bends down in the direction of gravity. In shoots, auxin has the opposite effect.
What is the difference between phototropism and gravitropism?
Phototropism is growth in response to light, and gravitropism is growth in response to gravity. Both are controlled by auxins. Shoots grow towards light and upwards, so they are positively phototropic and negatively gravitropic. Roots grow away from light and downwards, so they are negatively phototropic and positively gravitropic.
How are plant hormones used in agriculture?
Higher tier Growers use auxins as selective weedkillers, which make broad-leaved weeds grow too fast and die while cereal crops are unaffected, and in rooting powders for cuttings. Gibberellins are used to end seed dormancy, make flowers and fruits develop and produce seedless fruit. Ethene is used to ripen fruit after it has been transported.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026